Filter comprising one or more ducts

ABSTRACT

A membrane filter having one or more ducts is disclosed herein. Each of the ducts has a porous wall defining a lumen surrounded by helical grooves in the respective wall. The groove can be a single, double, or triple start groove defining a flow direction along the groove. The cross sectional area of the groove in the case of a single start groove, or aggregate cross sectional area of the groove in the case of a double or triple start groove, perpendicular to the flow direction along the lumen is within a range of 75% to 125% of the cross sectional area of the lumen. The helical grooves create vortices which reduce concentration polarization while increasing membrane area.

This invention relates to filters, particularly, though not exclusively,for medium-scale applications such as water purification or foodprocessing.

In my earlier WO 94/21362, I disclosed a way of enhancing theperformance of tubular membrane filters by introducing helical flowdeflectors to induce fluid mixing in the radial direction. One of theexamples disclosed in the application is of a rod with a helical grooveinserted concentrically within a tubular filtration membrane. Steadyfeed flow in the annular space between the impermeable helical insertand the concentric tubular permeable membrane provides excellent radialmixing. In a second example, the tubular filtration membrane liesconcentrically inside a casing containing a helical groove. Flowpatterns are created within the annular space between the impermeablecasing and the cylindrical permeable membrane, which ensure highfiltration performance and good mixing which prevents concentrationpolarisation. Both of these examples work well with a polymeric,permeable, membrane tube of diameter 12.5 mm, but it is difficult toscale up these apparatuses to provide membrane areas of the order of 1to 10 m² for larger applications without resorting to bulky andexpensive filter units.

Filters which have a large membrane area packed into a small volume forlarger applications are available commercially. One such filter providesa large number of parallel capillaries, in a highly porous block ofsupport material such as a ceramic, with a much tighter porous layer atthe wall of each capillary. The manifolding of the capillaries for feedfluid entry and exit is provided by the porous block. Filtrate passesthrough the capillary walls and then through the highly porous block.The filtrate is then collected in suitable channels at the outer surfaceof the porous block (FIG. 1). However, it is necessary to pump the feedflow through each capillary at velocities as high as 6 m/s in order toachieve reasonable mixing by turbulent flow and hence adequatefiltration performance. Although this design is space-saving it requiresvery high flow rates, and hence pumping costs (both capital and running)are correspondingly high. Furthermore, damage to delicate components inthe feed fluid, caused by turbulent flow is an additional disadvantageof this particular method.

One way of reducing the feed flow rate would be to place helical insertsin each tubular capillary in the porous ceramic block in a similarmanner to that used in WO 94/21362. However, these capillaries generallyhave a diameter of 4 mm of less, and it is difficult to constructhelical inserts of suitable geometry which are sufficiently robust andwhich are sufficiently rigid to avoid vibration and consequent damage tothe capillary walls.

GB-A-2223690 discloses a filter comprising one or more substantiallyunobstructed ducts with porous walls, the or each duct having a helicalgroove in the wall; and, according to the present invention, such afilter is characterised in that the groove is a single-, double- ortriple-start groove and the cross sectional area, or aggregate crosssectional area, of the groove, perpendicular to flow along the groove,is similar (as herein defined) to the cross sectional area of the lumen.

Unlike the prior art in which the porous membrane was of a cylindricalshape, the porous surface in this invention is of a helical shape. Thehelically grooved ducts thus provide excellent radial mixing and henceprevent concentration polarisation, but also provide increased membranearea, compared with conventional permeable membranes of circularcross-section. The present invention also avoids the high pumping costsof the prior art filter and the difficulty and expense of constructinghelical inserts which cause vibration and damage to cylindrical membranewalls.

The duct(s) is (are) preferably in a porous block of support material,and a denser porous surface may be formed at the wall(s) of the duct(s).

The porous block lends itself to the convenient formation of a largemember of ducts therein to create a large membrane area in a smallvolume for more efficient filtering. The filtrate can also beconveniently collected in a chamber or chambers at the outer surface ofthe porous block.

In use, the core-flow is through the central lumen of the duct with ahelical flow along the duct wall which produces a vortex within thehelical groove (FIG. 4). Similar flow patterns are described in my WO94/21362 except, of course, in the present invention the feed flow isinto an open tube rather than into an annular space.

The helical groove may be single-start, or may be double- ortriple-start, in order to reduce pressure drop along the ducts.Double-start helical grooves reduce the pressure drop along the duct bya factor of four and triple-start helical grooves reduce the pressuredrop by a factor of nine when compared with a corresponding duct with asingle-start helical groove, without compromising radial mixing. This isachieved because of the reduced length of each groove and the increasedtotal number of sub grooves.

The lumen of the duct may be up to 20 mm in diameter, but is preferablybetween 3 and 5 mm in diameter with adjacent turns separated by a land,eg, substantially 1 mm wide.

Experiments have shown that the cross sectional area, perpendicular tothe direction of flow along the helical groove, or the aggregate crosssectional area in the case of a multi-start groove, should be similar tothe cross sectional area of the central cylindrical lumen. By similar ismeant that the groove cross sectional area is within plus or minus 25%,and preferably within plus or minus 10%, of the lumen cross sectionalarea.

As seen in cross section perpendicular to the flow along the groove, theperipheral wall of the groove should be arcuate to promote a smooth flowpattern and an appropriate cross sectional shape is semi circular. Thusin the case of a groove cross section of semi circular shape, andapplying the preferred requirement that the groove cross sectional areaor aggregate cross sectional area is the same as that of the lumen, thenif the diameter of the semi circular cross section of the groove is cand the diameter of the lumen is d:${n \cdot \frac{( {\pi \quad c^{2}} )}{8}} = \frac{\pi \quad d^{2}}{4}$

when n is the number of groove starts.

Hence nc²=2d².

Thus for a single-start:

c={square root over (2)}d

for a double-start:

c=d

for a triple-start:

c= {square root over ({fraction (2/3)})} d

Thus if d=4 mm:

When n=1, c=5.6 mm and the groove depth is 2.83 mm.

When n=2, c=4 mm and the groove depth is 2 mm.

When n=3, c=3.28 mm and the groove depth is 1.64 mm.

In the accompanying drawings:

FIG. 1 is an example of a porous ceramic block containing capillariesaccording to the prior art;

FIG. 2 is a diagrammatic representation of how the filter may be used;

FIG. 3 is an axial section showing diagrammatically the typical geometryof a duct according to the present invention;

FIG. 4 illustrates the flow patterns present in the duct of FIG. 3;

FIG. 5 is a diagrammatic axial section of a duct having a double-startgroove;

FIG. 6 is a diagrammatic axial section of a duct having a triple-startgroove; and

FIG. 7 illustrates the method of making a filter in accordance with thepresent invention.

As shown in FIG. 2, a porous block 10 of sintered material is formedwith a number of longitudinal ducts 11. At an upstream end of the blockthese ducts open into an inlet manifold 12 and, at the downstream end,into an outlet manifold 13. The block is surrounded by an annularchamber 14 having an outlet 15. In use a fluid to be filtered is forcedby a pump P into the inlet manifold 12 and hence through the ducts 11.The filtrate passes out through the walls of the ducts and percolatesthrough the pores in the block 10 until it reaches the chamber 14, fromwhich it is recovered through the outlet 15. The concentrate of thefiltration passes into the outlet manifold 13 and hence through anoutlet 16.

FIGS. 3 and 4 show the internal geometry of a duct 11. This is shown ashaving a cylindrical lumen 17 surrounded by a single-start helicalgroove 18 of substantially semicircular cross-section, with adjacentturns separated by a land 19.

As described elsewhere, the wall of the duct may have a layer 20 of moredense porous material.

The secondary flow patterns produced when the duct is in use are shownin FIG. 4. Core flow 21 is through the lumen with the helical flow 22around the groove producing vortices 23, ensuring good mixing and highfiltration performance.

FIG. 5 shows a modification of the duct of FIG. 3, in which there is adouble-start groove 18(A) and 18(B), and FIG. 6 shows a duct having atriple-start groove;

The porous blocks with helically grooved ducts within them could be madeby a technique adapted from the well-known process for making ceramicfilters with cylindrical capillaries according to the prior art.

In this process, and with reference to FIG. 7, a tubular metal container24 has the required number of duct defining rods 26 fixed within it. Theduct defining rods of the present invention have helical formationsprojecting therefrom and are screwed into the top and bottom end platesof the metal container. Particulate clay 28 in dry or slurry form orglass or other ceramic or polymeric material is introduced into thespace between the duct defining rods 26. When filled, the container isheated in an oven to the temperature required to fire the clay or otherporous material. When the fired block has cooled, the duct defining rods26 are unscrewed from the porous block and the block is retracted fromthe metal container. The duct defining rods 26 and/or the metalcontainer 24 may be slightly tapered to improve release.

A denser porous surface may then be applied to the walls of the ducts.This can be done using a slurry of clay or other particles of muchsmaller size than those used to make the highly porous block.Dip-coating, spin-coating or slip-coating techniques are familiar toceramic and polymeric membrane manufacturers, any of which could be usedto apply the tighter membrane surface. The pores produced should have adiameter of about 0.2 μm for micro filtration, 0.02 μm for ultrafiltration or 0.002 μm for nano filtration.

All three types of pore size are useful in food and water processing.Dye processing would use mainly nano filtration. Ceramic materials canbe cleaned with aggressive chemicals and can be steam sterilised, bothimportant advantages for food or water processing.

An alternative method of manufacture would be to extrude ceramic tubeswith helical geometry. These could be extruded singly and then assembledin parallel into a stack. Alternatively, they could be co-extruded withrotating extruder heads.

As an alternative to ceramic or sintered materials, it should bepossible to inject open pore structural foam materials, such aspolyurethane, to form the porous block. Dip-coating of the tubular wallswith polymeric solutions, preferably prior to injection of thestructural foam, provides the selective membrane surfaces. Pore size isoften controlled either by dissolving particulates such as salt or bysolvent exchange (also known as phase inversion).

In the production of polymeric membranes it is often possible to providea skinned, or asymmetric structure, with the skin forming at a solidsurface, such as the surface of the duct-defining metal rods used in thepresent invention. Thus it should be possible to form the whole blockwith a single injection of polymeric open-pored foam. Although polymerfoams would be difficult to clean and sterilise they may offer bigsavings in cost, compared to ceramic membranes.

What is claimed is:
 1. A filter comprising one or more substantiallyunobstructed ducts (11), each said duct having a porous wall defining alumen surrounded by a helical groove in the respective wall; wherein thegroove is a single-, double- or triple-start groove defining a flowdirection along the groove; and a cross sectional area of the groove inthe case of a single-start groove, or aggregate cross sectional area ofthe groove in the case of a double or triple-start groove, perpendicularto the flow direction along the groove (18), is within a range of 75% to125% of the cross sectional area of the lumen (17).
 2. A filteraccording to claim 1, wherein said one or more substantiallyunobstructed ducts are in a porous block (10) of support material.
 3. Afilter according to claim 2, wherein a denser porous surface (20) isformed at the walls of the duct(s).
 4. A filter according to claim 2 orclaim 3, comprising means defining at least one chamber (14) at theouter surface of the porous block for collecting the filtrate.
 5. Afilter according to claim 2 wherein the helical groove (18) is ofsubstantially semi-circular cross-section.
 6. A filter according toclaim 3 wherein the helical groove (18) is of substantiallysemi-circular cross-section.
 7. A method of making a filter according toclaim 3, the method comprising providing a container with a plurality ofduct-defining rods fixed within it, the duct-defining rods havinghelical formations projecting therefrom; introducing material into spacebetween the duct-defining rods; forming the material into a porousblock; and removing the duct-defining rods and container from the porousblock.
 8. A filter according to claim 1, wherein the lumen (17) of theduct is up to 20 mm in diameter.
 9. A filter according to claim 8,wherein the lumen (17) is between 3 and 5 mm in diameter.
 10. A filteraccording to claim 9 wherein the helical groove (18) is of substantiallysemi-circular cross-section.
 11. A filter according to claim 8 whereinthe helical groove (18) is of substantially semi-circular cross-section.12. A filter according to claim 1, wherein the helical groove (18) is ofsubstantially semi-circular cross-section.
 13. A method of making afilter according to claim 12, the method comprising providing acontainer with a plurality of duct-defining rods fixed within it, theduct-defining rods having helical formations projecting therefrom;introducing material into space between the duct-defining rods andcontainer from the porous block.
 14. A method of making a filteraccording to claim 1, the method comprising providing a container with aplurality of duct-defining rods fixed within it, the duct-defining rodshaving helical formations projecting therefrom; introducing materialinto space between the duct-defining rods; forming the material into aporous block; and removing the duct-defining rods and container from theporous block.
 15. A method according to claim 14, further comprisingforming a denser porous surface at walls of the ducts.
 16. A filteraccording to claim 1 wherein the cross-sectional area, or aggregatecross-sectional area, of the groove in the case of a double ortriple-start groove, perpendicular to the flow direction along thegroove (18), is within a range of 90% to 110% of the cross-sectionalarea of the lumen (17).
 17. A method of making a filter according toclaim 16, the method comprising providing a container with a pluralityof duct-defining rods fixed within it, the duct-defining rods havinghelical formations projecting therefrom; introducing material into spacebetween the duct-defining rods; forming the material into a porousblock; and removing the duct-defining rods and container from the porousblock.
 18. A filter to claim 16, wherein the helical groove (18) is ofsubstantially semi-circular cross-section.